1. Field of the Invention
The present invention relates generally to a coding apparatus and method in a CDMA (Code Division Multiple Access) mobile communication system, and in particular, to an apparatus and method for coding a high speed downlink shared channel.
2. Description of the Related Art
In general, HSDPA (High Speed Downlink Packet Access) refers to a technique for transmitting data using HS-DSCH (High Speed-Downlink Shared Channel), a downlink data channel for supporting high-speed downlink packet transmission, and its associated control channels in a 3rd generation asynchronous mobile communication system. AMC (Adaptive Modulation and Coding), HARQ (Hybrid Automatic Retransmission Request), and FCS (Fast Cell Selection) techniques have been proposed in order to support the HSDPA. The AMC, HARQ and FCS techniques will be described herein below.
First, the AMC will be described. The AMC is one of the data transmission techniques for adaptively determining a modulation technique and a coding technique of a data channel according to a channel condition between a specific Node B and a UE (User Equipment), thus to increase the overall utilization efficiency of the Node B. Therefore, the AMC supports a plurality of modulation techniques and coding techniques, and modulates and codes a data channel signal by combining the modulation techniques and the coding techniques. Commonly, each combination of the modulation techniques and the coding techniques is called “MCS (Modulation and Coding Scheme),” and there are defined a plurality of MCS levels of #1 to #n according to the number of the MCSs. That is, the AMC adaptively determines an MCS level according to a channel condition between a UE and a Node B, thereby increasing the entire utilization efficiency of the Node B.
Next, the HARQ will be described, especially n-channel SAW HARQ (Stop and Wait Hybrid Automatic Retransmission Request). In the existing ARQ (Automatic Retransmission Request), an acknowledgement signal ACK and a retransmission packet are exchanged between a UE and a radio network controller (RNC) based on signaling from an upper layer. However, in the HSDPA, a part of the conventional MAC (Medium Access Control) functions in the RNC is transferred to a Node B, so that the ACK and a retransmission packet can be exchanged between a number of (“n”) UEs and MAC HS-DSCH of the Node B over a physical channel. The part of the MAC functions, transmitted to the Node B, is referred to as MAC-HSDSCH. In addition, n logical channels are formed to transmit several packets before ACK is received. The general ARQ does not transmit the next packet data or retransmit previously transmitted packet data until it receives ACK or NACK for the previously transmitted packet data. Therefore, in some cases, the ARQ must await ACK, although it can currently transmit the next packet data. However, in the n-channel SAW HARQ, the next packet data is continuously transmitted before ACK for the previous packet data is received, thereby increasing utilization efficiency of channels. That is, if n logical channels are established between a UE and a Node B, and the n logical channels can be identified by time and unique channel numbers, then the UE can recognize a channel over which packet data was received, and rearrange the received packets in the correct reception order, or soft-combine the received packets.
Finally, the FCS will be described. In the FCS, if a UE supporting the HSDPA (hereinafter referred to as “HSDPA UE”) enters a soft handover region, the HSDPA UE selects a Node B (or cell) having the best channel condition, and receives packets only from the selected Node B, thereby reducing the overall interference.
In order to apply the techniques applied to the HSDPA, the following control information is exchanged between a UE and a Node B (or cell). Since HS-DSCH fundamentally uses a multi-code transmission technique for high-speed transmission, information that must be transmitted from a Node B to a UE includes decoding-related information such as (i) channelization codes over which the HS-DSCH is transmitted, (ii) an MCS level used for the HS-DSCH and (iii) a code unit needed to analyze the HS-DSCH received, and information on the packets over which the HS-DSCH is transmitted. The information on the packets over which the HS-DSCH is transmitted may include HARQ information indicating a channel number, a packet number, and initial transmission or retransmission. Further, information that must be transmitted from a UE to a Node B includes ACK or NACK information for received packets, and information on a channel condition between the UE and the Node B to support the AMC and the FCS. In the FCS, if a cell having the best channel condition is changed, the UE transmits the corresponding information to the Node B so that a selected best cell can correctly transmit HS-DSCH to the UE.
FIG. 1 illustrates a method of transmitting downlink control information, downlink data, uplink control information and uplink data for the HSDPA. Referring to FIG. 1, a cell #1 101 is a cell providing an HSDPA service to a UE 111. Channels transmitted from the cell #1 101 to the UE 111 include (i) a high speed physical downlink shared channel (HS-PDSCH) for high-speed packet transmission, (ii) a downlink dedicated physical channel (DL_DPCH) for transmitting upper layer signaling, voice signal, and packet data and physical control information, which do not use HS-DSCH, and (iii) a shared control channel (SHCCH) for transmitting information required for analyzing the HS-PDSCH. Channels transmitted from the UE 111 to the cell #1 101 include an uplink dedicated physical channel (UL_DPCH) for transmitting upper layer signaling, voice signal, packet data and physical control information, and a secondary uplink dedicated physical control channel (S-UL_DPCCH) for transmitting control information for the HSDPA.
Structures of the uplink and downlink channels of FIG. 1 are illustrated in FIGS. 2A to 2C and 3A and 3B.
FIGS. 2A to 2C illustrate structures of downlink channels transmitted to an HSDPA UE, and FIGS. 3A and 3B illustrate structures of uplink channels transmitted by an HSDPA UE.
Specifically, FIG. 2A illustrates a structure of a high speed physical downlink shared channel (HS-PDSCH) transmitted from the cell #1 101 to the UE 111 in FIG. 1. The HS-PDSCH is transmitted at periods of 3 time slots 201 each having a length of 0.67 ms, and a data rate of the HS-PDSCH is determined based on an MCS level and the number of channelization codes in use. The channelization codes are used to distinguish between uplink and downlink channels in an asynchronous mobile communication system, and their length is defined as 4 to 512. The length of the channelization codes relates to a spreading factor (SF) of data.
FIG. 2B illustrates a structure of a downlink physical dedicated channel (DL_DPCH) transmitted from the cell #1 101 to the UE 111 in FIG. 1. The DL_DPCH is comprised of a downlink dedicated physical data channel (DL_DPDCH) and a downlink dedicated physical control channel (DL_DPCCH). In FIG. 2B, a first data part 212 and a second data part 215 are used to transmit upper layer signaling, voice data, and user data such as packet data which is not transmitted over the HS-PDSCH. A TPC (Transmit Power Control command) field 213 is used to transmit a power control command for controlling transmission power of an uplink channel transmitted from a UE to a cell. A TFCI field 214, a field for transmitting TFCI (Transmitted Format Combination Indicator), is used to transmit a data rate of the first data part 212 and the second data part 215, a channel configuration type, and channel decoding information. A Pilot field 216 is used by a UE to estimate a condition of a downlink channel transmitted from a cell to the UE. The first data part 212 to the Pilot field 216 constitute one time slot having a 2,560-chip length, and 15 such time slots constitute one radio frame having a 10 ms length. The radio frame is a basic physical transmission unit used in 3GPP (3rd Generation Partnership Project), a standard for an asynchronous mobile communication system.
FIG. 2C illustrates a structure of a shared control channel (SHCCH) transmitted from the cell #1 101 to the UE 111 in FIG. 1. The SHCCH, a channel for transmitting control information needed to receive HS-DSCH transmitted from the cell #1 101 to the UE 111, is a shared channel which is alternately received by UEs in a corresponding Node B (or cell) supporting an HSDPA service. The SHCCH can transmit control information required for receiving HS-DSCH, to one UE or a plurality of UEs at a certain point of time. In FIG. 2C, SHCCH 221, comprised of 3 time slots, transmits TFRI (Transmitted Format Resource Indicator) 223 and HARQ information 225 for a period of the 3 time slots. The TFRI 223 includes an MCS level used in the HS-DSCH, the number and type of channelization codes, and information required for decoding the HS-DSCH. The HARQ information 225 indicates a channel number transmitted in an n-channel SAW HARQ system supporting the HSDPA, and further indicates whether a packet to be transmitted over HS-PDSCH is an initial transmission packet or a retransmission packet retransmitted due to an error. The SHCCH is transmitted from a cell providing an HSDPA service to only a UE receiving the HSDPA service.
FIGS. 3A and 3B illustrate uplink channels transmitted from a UE to a cell in reply to the downlink channels illustrated in FIGS. 2A to 2C. Specifically, FIG. 3A illustrates an uplink dedicated physical channel (UL_DPCH) 311, which is comprised of an uplink dedicated physical data channel (UL_DPDCH) and an uplink dedicated physical control channel (UL_DPCCH). The UL_DPDCH is used to transmit uplink control information or user information from a UE to a cell or cells, and the UL_DPCCH is used to transmit physical control information from the UE to the cell. The UL_DPCCH is basically identical to the DL_DPCCH in function of individual fields. The UL_DPCCH and UL_DPDCH are channel-coded with different channelzation codes, and transmitted over I and Q channels of QPSK (Quadrature Phase Shift Keying), respectively. A basic transmission unit of the UL_DPCH is 10 ms radio frame, and the 10 ms radio frame is comprised of 15 time slots. Each of the time slots includes a Pilot field 312, a TFCI field 313, an FBI field 314, and a TPC field 315. The Pilot field 312 enables a cell or cells receiving the UL_DPCH to estimate a condition of an uplink channel transmitted from a UE to the cell. The TFCI field 313, a field (or channel) for transmitting TFCI of the UL_DPDCH, indicates a channelization code and a data rate used for the UL_DPDCH, decoding information, and the type of data transmitted over the UL_DPDCH. The FBI (Feedback Information) field 314 is used to transmit closed-loop antenna control information, in the case where closed-loop antenna-based transmission is used for downlink transmission from a Node B or cell to a UE. In addition, the FBI field 314 is used to transmit control information for supporting SSDT (Site Selection Diversity Transmission), in the case where a UE, though it is located in a soft handover region, supports the SSDT for receiving DL_DPDCH from only one Node B having a good downlink channel condition. The SSDT is a technique which has evolved into FCS and newly introduced to the HSDPA. The TPC field 315 is used to transmit a power control command for controlling transmission power of downlink channels transmitted from a Node B (or cell) to a UE.
FIG. 3B illustrates a secondary uplink dedicated physical control channel (S-UL_DPCCH) for transmitting control information from an HSDPA UE to a cell. As stated above, the HSDPA UE must transmit ACK or NACK information for a received packet to a Node B (or cell) which has transmitted HSDPA data, and can transmit channel measurement information for selecting an MCS level or a best cell. Such information is transmitted over the S-UL_DPCCH. The S-UL_DPCCH may transmit ACK/NACK information 323 and a channel quality indicator (CQI) 325 for a 3-time slot period. The reason for introducing the S-UL_DPCCH to the HSDPA is to maintain compatibility with the existing 3GPP communication system by introducing a new channel for the HSDPA service without modifying a structure of the conventional UL_DPCH. The S-UL_DPCCH is transmitted only to a cell which transmits HSDPA data.
The CQI 325, information indicating a measured quality of a channel transmitted from a Node B transmitting HSDPA data to a UE receiving the HSDPA data, is transmitted from the UE to the Node B as information for selecting an MCS level or selecting the best cell in the FCS. The CQI may be calculated as a ratio of a signal to noise or interference, applied to a UE. An amount of the CQI information is variable according to an interval at which a UE reports a range of the signal-to-noise (or interference). ratio to a Node B in communication with the UE. For example, if the UE reports the signal-to-noise ratio to the Node B at intervals of 1 dB from −20 dB to +12 dB, the information amount becomes 32 (i.e., the information is transmitted 32 times). The CQI is very important control information that is used by a Node B transmitting HSDPA data to determine a rate of data received by a UE receiving the HSDPA data. If the CQI has an error, a Node B supporting the HSDPA incorrectly selects an MCS level. As a result, although the UE receiving HSDPA data has a good channel condition, the Node B transmits data at a low data rate, thus reducing transmission efficiency. Alternatively, although the UE has a poor channel condition, the Node B may transmit data at a high data rate, causing an error during data reception at the UE.